Traditionally, two wires, one for the signal and another one for the ground, are necessary to realize communication between two terminals. Such transmission systems are termed two-wire circuits, and may use, for example, a twisted pair, or a coaxial cable as a transmission medium. Such systems are capable of transmitting signals in one direction at a time, possibly alternating between different directions, with buffering at the two ends. A pair of two-wire circuits form a four-wire circuit, which is already capable of simultaneous, uninterrupted full-duplex communication. Another family of communication tools, fiber optic devices, are capable of transmitting information in both directions across a single glass fiber at the same time.
The telecommunication and computer industries have made numerous successful attempts to increase the amount of information a wire pair can transfer, or, to decrease the number of wires necessary to transmit information at a given rate. For instance, U.S. Pat. Nos. 6,803,790 and 6,791,356 propose a bidirectional digital port with a clock channel used for synchronization. The port allows digital circuits to communicate with each other simultaneously. Further examples include the full-duplex token-ring based telephone system described in U.S. Pat. No. 6,611,537, and the digital telephones described in U.S. Pat. Nos. 4,726,054 and 6,925,091.
Single-wire digital telephone systems are presented in U.S. Pat. Nos. 4,731,821 and 4,835,765. U.S. Pat. No. 5,451,923 describes bidirectional digital communication over a pair of conductors. The same pair of conductors are also used for instrument diagnostics and control. Nowadays, even some refrigerators are equipped with digital bidirectional communication systems, as shown in U.S. Pat. No. 5,485,397. The communication process described in U.S. Pat. No. 5,485,397 is not simultaneous, however.
The time-multiplexing of signals sent through numerous wires makes it possible to reduce the number of wires, at the expense of transmission rate. Multiplexing is also possible in frequency domain, whereby different frequency modulated signals are sent on different carrier frequencies. It is also customary to divide messages into packets, then transmit the packets at a high rate, often utilizing the same transmission medium for sending packets of several communication processes one after another, and finaly, to assemble the original message from the packets at the destination. Such systems are often implemented in land-based digital phone networks.
Several circuit arrangements capable of realizing simultaneous, full-duplex communication between two or more terminals have been patented in the past. Most of these communicate digital high or low voltage levels only. Also, most of these systems are relatively complex and require various filters, which lowers the noise immunity of the system. A digital, full-duplex serial bus capable of sending data in packets is disclosed in U.S. Pat. No. 6,411,628. Full-duplex communication can also be realized across the three-phase electrical power lines, as shown in U.S. Pat. No. 6,154,488. The system uses more than one wire for communication. U.S. Pat. No. 5,515,038 also presents a digital communication system that uses the three phase power lines.
A C-MOS bi-directional, differential communications link is described in U.S. Pat. No. 5,666,354. This system is directed to digital communications. The full-duplex computer network modem prescribed by CCITT Recommendation V.26 ter. is also capable of realizing simultaneous digital full-duplex communication.
U.S. Pat. Nos. 6,373,908 and 6,744,831 describe a simultaneous, full-duplex communication system developed for digital signals. The signals sent through the twisted pair connecting two terminals are analog, and necessitate the use of a transformer at each end of the twisted pair. The transformer is typically large, expensive, heavy, and not possible to integrate into a chip.
U.S. Pat. No. 5,657,324 describes a bidirectional communication system, primarily geared to communicate digital signals across a single cable in two directions simultaneously. The system works with positive voltage signals only. The transceivers in a two-terminal network are called master and slave, and their functions are not symmetrical. This disadvantage is manifested when the transceivers are connected into a network. In that case, all slaves listen to the master simultaneously, but only one slave can talk to the master at a time. The slaves cannot talk to each other.